Method of completing wells



MalCh 27, 1962 A. J. TEPLITZ METHOD oF COMPLETING WELLS Filed April l5, 1955 IN V EN TOR.

United States Patent 3,026,936 METHOD OF COMPLETING WELLS Abraham J. Teplitz, Penn Township, Allegheny County,

Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Apr. 13, 1955, Ser. No. 501,063 12 Claims. (Cl. 16o- 23) This invention relates to a method of completing fluid producing wells and more particularly to a method of completing wells by which a permeable cement sheath is formed in the producing formation. This application is a continuation-impart of my copending patent application, Serial No. 424,862, filed April 22, 1954, now abandoned.

The method of the invention is especially useful for completing wells in poorly consolidated formations where sand control is a problem. In producing oil, gas, or other fluids from poorly consolidated formations such as unconsolidated sands, it is desired to avoid the production of sand with the fluids in order to prevent clogging of the well bore which results in costly shutdown and to avoid damage to pumps and other well equipment. A number of procedures for sand control have been employed including the use `of various types of strainers in the producing formation; for example, wire-wrapped slotted pipes, gravel packs, and the like. The prior art means for sand control have not been entirely satisfactory. Abrasion caused by sand particles enlarges the openings in the wire-wrapped slotted pipe and allows the production `of substantial quantities of sand which accumulate in the well bore, stopping production and necessitating an expensive clean-out process. The openings in the pipe are also subject to clogging by sand with resulting decrease in production. Gravel packs are also subject to clogging. This is due to the fact that it is difficult to place the gravel in the well without forming voids in some places in the pack. Such voids allow shifting of the gravel during production and consequent movement of sand from the surrounding formation which clogs fluid passages in the pack and stops or reduces production.

In completing Wells in consolidated producing formations certain problems can occur when conventional methods of completing wells by gun perforation are practiced. Recent evidence indicates that when bullets or shaped charges from a gun or jet perforator are fired through a well casing and into a consolidated producing formation, drilling mud or other solids may be forced into the perforations. When this occurs the productivity of the formation is poor. Tests show that the mu-d or other finely divided solids vcan form a solid plug in the perforation. In some instances mud is even forced into the pores of the consolidated formation and reduces permeability. The method of the present invention can avoid these problems of low productivity which can occur in wells completed in consolidated formations by gun perforation.

In accor-dance with the present invention, many of the disadvantages of prior art methods of well completion are avoided. In general, the invention comprises a method of completing a well in which a sheath of cementitious material is formed in the annular space between the well casing and the producing formation wall. Thereafter, a stress is applied to the set cementitious sheath sufficient to produce small fractures in the cementitious material which provide passages for fluids from the producing formation to perforations in the well casing. This stress must not be enough to destroy the general outline of the cement sheath or to form fractures large enough to serve as passages for substantial amounts of sand from an unconsolidated producing formation. When the well is completed in this manner the fractured cement sheath permits fluids to iiow into openings formed in the well 3,025,936 Patented Mar. 27, 1962 y casing but excludes sand and provides support for the surrounding formation. When the well is completed in the manner of the invention in a consolidated producing formation, the low productivity resulting from mud plugging of -bullet perforations in Wells completed in the conventional way is avoided.

The method of completing a well of the present invention will be described with reference to the accompanying drawing. In the drawing,

FIGURE 1 is a diagrammatic view in vertical section of a producing formation in an oil Well during a phase in the completion of the well by the method of the invention; FIGURE 2 is a similar view of the well after perforations in the tubular casing and fractures in the surrounding cement sheath have been formed.

FIGURE l shows, by way of example, a well extending through a consolidated formation 10, an unconsolidated oil sand 11 and into a consolidated formation 12. The well is cased throughout the producing formation with a tubular casing 14, which is preferably steel casing pipe commonly used in the art. FIGURE 1 shows casing 14 as an unperforated pipe which is to be perforated after it is placed in the well, as described in more detail hereinafter. However, if desired, the pipe can be perforated before it is placed in the well. Conventional shop-perforated or slotted pipes, screen pipes and the like can be used. For convenience the term casing is used in the specification and claims, in a sense perhaps somewhat broader than usual, to designate any of the tubular means used for lining wells, whether perforated before or after being placed in the Well, about which a cementitious sheath can be formed in the method of the invention as describe-d hereinafter.

The casing 14 is surrounded by a cementitious sheath 15, formed preferably from a slurry of neat Portland cement. Care is taken to eliminate voids or channels from the cement sheath 15 before it sets, for example, by using scratchers and centralizers in the cementing operation. After the cement has set, the well is ready for the phase of the method of the invention in which fractures are formed in the cement sheath and perforations are formed in the tubular casing if they have not been already formed.

The fracturing of the cement is accomplished in the method of the invention by applying a stress to the sheath so that it is strained and a network of cracks is produced i in the cement. The stress can be applied in a number of different Ways in accordance with the invention, several of which will be described. The structure of the well afterv the cement sheath has been fractured and perforations have been formed in the tubular casing is illustrated diagrammatically in FIGURE 2. The structure comprises the casing 14 having openings 18 therein and the surrounding cementitious sheath 15 having a network of small fractures or cracks which provide passages for fluids from the producing formation 11 into the openings in the casing.

When a well is completed by the method of the invention, in which a limited stress is applied to the cement sheath in the producing zone while it is confined between the well casing and the formation wall, many small fractures form in the portion of the sheath to which the stress is applied but the sheath retains its general outlinel as shown in FIGURE 2 and continues to support the surrounding sand formation. Consequently, the shifting of sand which results in the clogging of conventional well strainers in oil sands is prevented or greatly reduced. The small fractures in the cement allow fluids to enter the well from the producing formation but prevent the production of sand of any substantial particle size. The greatest pressure gradient during production from a well completed by the method of the invention occurs as the fluids pass through the fractures in the cement rather than, as in conventionally completed Wells, in the sand formation. Consequently, the forces tending to disturb the sand formation and cause movement of sand are considerably smaller than in conventional well structures.

I have described my method with particular reference to well completion in a poorly consolidated sand formation such as shown in the drawing. However, the procedure is substantially the same when applied to completion of a well in a consolidated or competent producing formation. The casing (preferably unperforated) is cemented in the well opposite the consolidated producing formation. After the cement has hardened and set, a fracturing stress is applied to the cement from within the casing in one of the suitable ways described hereinafter. After the cement has been fractured, the casing, if not preperforated, is perforated in some suitable way that does not have the disadvantage of forcing mud or other solids under high pressure into the producing formation. Most suitably the casing is perforated by a conventional knife type casing perforator. When the casing is perforated, oil or other iluid can then flow from the producing formation through the fractures in the cement sheath and into the casing.

As I have mentioned, in my method of completing a well the cement sheath can be fractured in a number of different ways. One suitable modification of the method can be described with reference to FIGURE 1 of the drawing. A gun perforating device 20, of conventional design, is lowered through the cemented casing 14 to the level of the producing formation. The tiring barrels of the gun are loaded in a manner which is adapted to cause considerably less penetration by the projectiles than is accomplished in the conventional use of such devices.

The Igun 20, for example, can be loaded with nonpenetrating bullets which when fired against the inner wall of the cemented casing apply a hammering force to the casing and a stress to the surrounding sheath suflicient to form the desired fractures in the sheath. In this operation the casing can be preperforated or perforated after the firing of the bullets, for example, with a mechanical casing perforator of the knife type.

Non-penetrating bullets with which the gun can be loaded include soft-nosed lead bullets and cylindrical or unpointed steel bullets. Such bullets when fired against the casing wall with a properly reduced propellant charge do not pierce the casing but the shock of their striking against the casing causes fracturing of the surrounding cement sheath.

In. another modification of my method, bullets can be usedE so as to pierce the casing and fracture the s... rounding cement without penetrating completely through the cement and without penetrating the formation. This result can be achieved by using conventional pointed steel bullets and a propellant charge which is sufciently small to prevent excessive penetration. A particularly suitable method for accomplishing this result is disclosed in the copending application of Theodore A. Kibby, Serial No. 450,630,.iiled August 18, 1954, abandoned in favor of a continuation-in-part application, Serial No. 526,171, which issued as United States Patent No. 2,896,718. In this method there is used a bullet of a structure that is particularly adapted to limit penetration. This bullet has a conical nose which has a base of smaller diameter than the cylindrical body of the bullet. An annular shoulder is formed where the conical nose joins the cylindrical body of the bullet. When the bullet is fired with a smaller charge than normally used for gun perforating with a conventional bullet of the same caliber, the conical point (which is longer than the thickness of the casing) pierces a hole in the casing and the shoulder impedes further penetration. The bullet imparts to the casing asharp physical shock which causes small fractures. to form in the surrounding cement. Furthermore, the perforating of the casing and the. fracturing of the cement are performed substantially simultaneously. Usually the shoulder causes the bullet to rebound and leave a clean hole after piercing the casing but if bullets should remain in the casing, a Well scraper can be used to clear the perforations.

As indicated above, the propellant charge with which non-penetrating bullets are red in the method of theV invention is smaller than normally used for casing perforation with bullets of similar size. For example, with bullets of from about 1A; to 5%: inch in diameter, from about 1A to the usual charge gives the desired results.

In another modification of the invention, the device which is lowered into the Well to apply stress to the cement sheath is a mechanical hammering device instead of gun perforator 20 of FIGURE l. The hammering device is used to direct sharp blows against the inner walls of the casing with force sufficient to fracture to surrounding cement. The casing can be either preperforated or perforated after the hammering operation, for example, by means of a knife-type casing perforator.

It is also possible, in accordance with the invention, to fracture the cement sheath by detonating an explosive charge within the tubular casing. An explosive charge of the proper amount will bulge or deform the steel casing without rupturing it so that a stress will be applied to the surrounding sheath to cause fractures of the desired kind in the cement.

A particularly good explosive for use in the method of the invention is explosive cord such as the material known commercially as Primacord. This cord explodes at a rate of about 20,000 feet per second. The cord is 1A inch in diameter and comprises an explosive core of PETN (pentaerythritol tetranitrate) and a waterproofed textile covering. It is tough and flexible and relatively insensitive to ordinary shock, friction, fire, or temperatures encountered in oil wells. Therefore, it is safe to handle and resistant to premature explosion when used in the method of the invention. It is detonated by an electric blasting cap which can be actuated from ground level. While Primacord is preferred, other explosive cords having properties such as those described can also be used in the method of the invention. The casing can be preperforated or perforated after the explosion.

Another advantageous method of forming fractures in the cement sheath is by mechanically applying a deforming stress to the tubular casing such stress however being less than the elastic limit of the casing so that the casing will not be ruptured. For example, there can be lowered into the casing opposite the producing formation a device which will apply an outward force to expand and deform the casing. One such device, which is described in the copending patent application of Abraham J. Teplitz and Forest F. Versaw, Serial No. 445,842, tiled July 26, 1954, now U.S. Patent No. 2,884,066, comprises a cylindrical member having radially extending plungers or pistons connecting with an interior hydraulic chamber. Within the chamber a vertically moving piston, actuated from ground level, applies hydraulic force to the horizontal pistons and forces them against the inner wall of thewell casing, thus expanding and deforming the casing. The cement sheath is strained when the casing is deformed and fractures of the desired type are produced. The casing is perforated in any desired manner before or after the fracturing operation. As an alternative to the mechanical expansion of the casing it is possible, in accordance withV the invention, to expand and deform the casing with hydraulic pressure. A hydraulic uid is conlined in the casing opposite the producing formation between suitably placed packers. A sufficient pressure is applied to the confined uid to expand and deform thev casing and thereby fracture the surrounding cement sheath. However, the pressure must not be so great as to cause the casing to rupture.

If itis found that the fractures formed in the cement by. one of theA described methods are not permeable enough to give the desired production of iiuids, they can be made more permeable by any of several well known methods, for example, by treating the cement with a chemical agent such as hydrochloric acid, or with a fracturing uid which is forced into the fractures in the cement under pressure.

In practicing the various modifications of the method of the invention, the amount of stress to be applied is determined by the results desired. Thus, for example, with the gun perforating device, when it is desired to perforate the casing and fracture the cement simultaneously, the propellant charge for the bullets will be somewhat greater than when it is desired merely to apply a hammering force against the casing to fracture the cement without piercing the casing. In any event, the charge is so limited that the projectiles do not penetrate excessively into the cement sheath and form large openings through to the formation wall.

Whatever means is chosen yfor fracturing the cement sheath and whether it be fractured before, after, or simultaneously with the perforating of the casing, the force or stress applied to the sheath should be suiiicient to fracture the cement and produce small passages for fluids from the producing formation into openings in the casing. But the force must not be so great as to produce fractures which would 'pass substantial quantities o-f sand into the casing or so great as to destroy the general shape of the sheath so that its function of supporting the sand formation could not be performed. When the well is being completed in a consolidated producing formation, the force must not be of the kind or magnitude that would cause mud or other solids to be forced into the producing formation to such an extent as to excessively reduce its permeability.

The results obtainable by the method of the invention are illustrated by an operation in which a cement sheath surounding an 8 inch casing was fractured by mechanically deforming the casing. A length of 8 inch casing was centered in a jacket 141/2 inches in diameter and the annular space between the casing pipe and the jacket was filled with cement. When the cement had set a casing deforming apparatus was lowered into the casing. This apparatus was in effect a hydraulic jack having horizontal pistons slidably mounted in cylinders extending radially from the device. The horizontal pistons, by hydraulic action, were forced outwardly against the inner walls of the casing to expand the pipe. The pistons were then retracted and the jack was rotated through an angle of approximately 72. This procedure of expanding, retracting and rotating the jack was repeated until the pipe was expanded in directions. The operation was then repeated at a lower level in the pipe. The outer jacket was removed and it was observed that the cement had been thoroughly fractured by the expansions of the jack. Thus, the cement sheath was fractured and made permeable to fluids by applying a force from Within the casing without rupturing the casing.

In the foregoing description I have discussed only production from a single level. However, an important advantage of my method of completing a well is that dual production can be accomplished. That is, a well drilled through two or more producing formations can be completed at each level by my method and oil can be produced simultaneously from each of the formations. This characteristic of my method is, of course, an important advantage over conventional well completion methods, such as the gravel packing method, which cannot be applied to widely separated producing zones in a single well.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

l. A method of establishing fluid communication in a well between a producing formation and a casing consisting of cementing a tubular casing in the well in the producing formation so as to form a cement sheath between the casing and the producing formation that is substantially free of voids, forming openings in the casing, and applying a stress to the cement sheath by hammering the inner wall of said tubular casing opposite the producing formation, said stress being sufficient to form a network of small cracks extending through the cement sheath from the producing formation to the openings in the casing but insuliicient to destroy the general outline of the cement sheath, said network of cracks constituting passages affording fluid communication between the producing formation and the openings in the casing.

2. A method of establishing iiuid communication in a well between a producing formation and a casing consisting of cementing a tubular casing in the well in the producing formation so as to form a cement sheath between the casing and the producing formation that is substantially free of voids, forming openings in the casing, and applying a stress to the cement sheath by detonating an explosive charge within said tubular casing opposite the producing formation, said stress being sufficient to form a network of small cracks extending through the cement sheath from the producing formation to the openings in the casing but insuflicient to destroy the general outline of the cement sheath, said network of cracks constituting passages affording fluid communication between the producing formation and the openings in the casing.

3. A method of establishing fluid communication in a well between a producing formation and a casing consisting of cementing a tubular casing in the well in the producing formation so as to form a cement sheath between the casing and the producing formation that is substantially free of voids, and applying a stress to the cement sheath by tiring a projectile against the inner wall of said tubular casing opposite the producing formation, said stress being sucient to `form a network of small cracks extending through the cement sheath from the producing formation to the openings in the casing but insufficient to destroy the general outline of the cement sheath, said network of cracks constituting passages aording fluid communication between the producing formation and the openings in the casing.

4. A method of establishing fiuid communication in a well between a producing formation and a casing consisting of cementing a tubular casing in the well in the producing formation so as to form a cement sheath between the casing and the producing formation that is substantially free of voids, forming openings in the casing, and applying a stress to the cement sheath by forcing plungers against the inner wall of said tubular casing opposite the producing formation, said stress being sufficient to form a network of small cracks extending through the cement sheath from the producing formation -to the openings in the casing -but insuiiicient to destroy the general outline of the cement sheath, said network of cracks constituting passages affording fluid communication between the producing formation and the openings in the casing.

S. A method of completing and producing a well comprising installing tubular casing in a producing formation of the well, forming a cement sheath between the casing and the producing formation, applying a stress to the cement sheath from within the casing opposite the producing formation, said stress being sufficient to form a network of small cracks extending through the cement sheath from the producing formation to the casing but insufficient to destroy the general outline of the cement sheath, said network of cracks constituting passages affording the sole fluid communication from the producing formation to the casing, providing openings in the casing communicating with the network of cracks, and producing fluids from the formation solely through the network of cracks and the openings in the casing.

6. The method of claim 5, wherein unconsolidated -formation solids are substantially excluded from entering the casing and the network of cracks during said producing step by the cracks being sufficiently limited in size to deny such entry.

7. The method of claim 5, wherein said stress is applied by ring a projectile against the casing, said projectile being tired with sufficient force to pierce the casing without the projectile passing entirely through the casing.

8. The method of claim 5, wherein said stress is applied by hammering the inner wall of the casing.

9. The method of claim 5, wherein said stress is applied by detonating an explosive charge within the casing.

10. The method of claim 5, wherein said stress is applied by forcing plungers against the inner wall of the casing.

1l. A method of completing and producing a well comprising installing tubular casing in a producing formation of the well, forming a cement sheath between the casing and the producing formation, applying a stress to the cement sheath from within the casing, said stress being suflicient to form a network of small cracks extending through the cement sheath from the producing formation to the casing but insufcient to destroy the general outline of the cement sheath, said network of cracks constituting passages affording the sole uid communication from the producing formation to the casing, providing openings in the casing communicating with the network of cracks, said stress being applied and also said openings being provided by firing projectiles adapted to pierce the casing against the inner wall of the casing, removing the projectiles from the openings resulting from their piercing of the casing, and producing fluids from the formation solely through the network of cracks and the openings in the casing.

12. A method of completing and producing a well comprising installing tubular casing in a producing formation of the well, forming a cement sheath between the casing and the producing formation, applying a stress to the cement sheath from within the casing opposite the producing formation, said stress being sufcient to form a network of small cracks extending through the cement sheath from the producing formation to the casing but insuicient to destroy the general outline of the cement sheath, enlarging said cracks without destroying the general outline of the cement sheath, said enlarged cracks being suiciently limited in size throughout their extent to prevent substantially the passage of unconsolidated formation solids therethrough, said network of enlarged cracks constituting passages affording the sole fluid communication from the producing formation to the casing, providing openings in the casing communicating with the network of enlarged cracks, and producing fluids from the formation solely through the network of enlarged cracks and the openings in the casing.

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